Mechanistic Force and Burr Modeling in High-speed Microdrilling of Ti6Al4V

Microdrilling can be used to create micro holes (<100 μm) in turbine blades, guide vanes and medical implants made of Ti6Al4V which has high strength to weight ratio and excellent biocompatibility. Microdrills have limited flexural stiffness which can lead to catastrophic tool failure. The undesirable microburr formation is another major impediment. To counter this lack of stiffness, high rotational speeds can be used to reduce the chip load and forces. It is expected that lower forces could reduce the burr formation as well. It may be noted that macro-drilling models have been reported in the literature but these models do not adequately represent the microdrilling regime where the chip thickness is of the order of microns and cutting velocities are very high. Consequently, in this paper, a mechanistic drilling force model based on specific normal and friction forces has been developed as a function of the feed rate and spindle speeds. The forces were determined by summation of specific cutting forces over the cutting edge and using oblique cutting model to evaluate forces in the feed direction. Experiments have been carried out to validate the force model using microdrills of 300 μm, spindle speeds of 60,000, 80000 and 100000 rpm and the feed of 0.0167, 0.025 and 0.03 μm/rev. Specific normal forces (Kn) was calculated for different feed rate and cutting velocity. The model is in good agreement with the experimental results. The burr model has also been developed which is based on energy balance where the net deformation work is used in shear, elongation and bending. The burr height prediction model is in good agreement with the experimental results.